1. Field of the Invention
The present invention relates to monitoring voltages, and more particularly to monitoring power supply voltage levels in low power, portable products.
2. Background Information
Monitoring power supply voltage levels to ensure proper operating voltage levels is becoming more important as integrated circuit power dissipation levels decrease forcing power supply levels and current draws to decrease. Tighter operating supply voltage levels for ensuring sufficient supply voltages are required, and detecting when a supply voltage is within or without acceptable limits becomes more demanding.
There have been a number of recent patents addressing this issue. One such patent is U.S. Pat. No. 6,750,683 to McClure et al., owned by STMicroelectronics, Inc. and entitled “Power Supply Detection Circuitry and Method.” This invention includes a feedback connection that provides a hysteresis to prevent oscillations. The invention uses a band-gap as a reference level for comparing the rising and falling power supply voltage level.
Another patent is U.S. Pat. No. 6,759,852 to Samad, owned by Xilinx, Inc. and entitled, “VDD Detection Path in Power-up Circuit.” FIG. 1 illustrates this particular invention. Q1 is a p-type CMOS device that is fabricated with a threshold of about 430 mV. The resistor R1 and the threshold of Q2 and the resistor R2 are fabricated to trigger a Schmitt trigger at a value T1, when Vdd reaches a proper level. When this occurs, EN signals the circuitry 10 powered by VDD to begin operation. As VDD decreases, the level into the Schmitt trigger decreases until a value T2 is reached (T2<T1) whereupon the EN is made false (low) and the circuitry is informed that the power supply is too low. It is assumed the circuitry 10 would take suitable steps in such a condition.
The prior art circuits require special devices, like band-gaps, which are often not power efficient over the required range of operating conditions. Also, since the prior art circuits are powered by the power source being monitored, the inter-dependence leads to inadequate noise margins along with power inefficiencies over the typical operating conditions for the circuits.
The prior art did not realize that portable, low power devices that often receive an additional power supply from the cable interface to which they are attached or through an antenna (as in RFID, radio frequency identification, devices). These particular devices dissipate less than 100 microwatts and are often attached via USB (universal serial bus) ports. In such applications, low power operation, power efficiency and adequate noise margins are important for reliable operation. The present invention is directed to these devices, but may find advantageous use with other circuits and systems.